proof of purchase

Welcome to the Event, @ianleonard!

Here is a great resource from the IRS about documentation of business expenses:

https://www.irs.gov/businesses/small-businesses-self-employed/what-kind-of-records-should-i-keep#:~:text=Documents%20for%20expenses%20include%20the,Account%20statements 

Receipts, account statements, invoices are all good methods. 

You will use these when you are figuring out your Profit and Loss Statement for your business at tax time. 

The IRS may ask for supporting documentation at any time - whether for an audit or just informational.

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Records for verification on your purchases can come in different formats.  Printed Sales Receipts, e-receipts, Digital Invoices, Credit Card Statements and Voucher Copies.  Keeping track of your information is key and is two parts; first organizing your receipts to where you can access the information and second have the transactions recorded in an organized in an accounting system that will tie everything together to provide totals. 

Specialized items like custom created or high value will generally need more detail.  The purchase/sale of a property will need the sales transaction records since you will need not only the sales price, but all the additional costs related to the purchase/sale.  The purchase of a large piece of equipment will need not only the amount paid for the item but also the cost to put the equipment in place and in use.